Return prime factors from int as input as an array

I need to code an algorithm which takes an int, gets its prime factors, puts them into an array and returns them.

My code is the following.

public static int[] primfaktorzerlegung(int zahl) {
   int d=1; //this is the length of the array

   int[] result = new int[d]; //array has to be returned

   List<Integer> factors = new ArrayList<Integer>();

   for(int factor = 2; factor <= zahl; factor++) {
       while(zahl % factor == 0) {
           factors.add(factor);
           zahl = zahl / factor;
       }
   }

   for(int i : factors){ //trying to get every number of the arraylist
       int z = i;          
       result[d] = z; //trying to put the numbers of the arraylist into the array result
       d++;           //makes the array dimension one higher
   }
   return result; //returns the array

}

I get the following error:

Error: java.lang.ArrayIndexOutOfBoundsException:
Index 1 out of bounds for length 1
at: result[d] = z;

What could be the cause?

Solution

You are not actually increasing the size of the array by incrementing d. Once you have allocated the array, its size is fixed.

You can do this instead:

public static int[] primes(int number) {
    List<Integer> factors = new ArrayList<>();
    for(int factor = 2; factor <= number; factor++) {
        while (number % factor == 0) {
            factors.add(factor);
            number = number / factor;
        }
    }
    return factors.stream().mapToInt(n -> n.intValue()).toArray();
}

The stream() method exposes the ArrayList as a Stream, which allows you do use nice methods to manipulate collections. One of them is mapToInt, which allows you do apply a function to each element in the stream. You apply the function that takes an n and returns whatever comes in the body (the part after the ->). In particular, since you are putting together a collection of boxed Integers, you have to unbox them into ints (more on boxing here). The intValue() method does exactly that. Finally, you return an int[] by calling toArray() on it. In practice, you are saying: apply intValue() to each item in the list and return the resulting array.

Note that I took your main logic as is, I didn't go into the correctness of how you are computing prime factors.